Do not call 'cmp' with non-existant -q flag.

I cannot find when that flag was removed if it ever existed, I can find
nothing about it in the ChangeLog and NEWS file of GNU diff.  The current
flag is -s aka --quiet aka --silent, so let's use -s, assuming it is a
portable flag.  Feel free to lart me with a POSIX bible if needed.

Signed-off-by: Yann Dirson <ydirson@altern.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>